Infrared (IR) singularities in massless gauge theories are known since the foundation of quantum field theories. The root of this problem can be tracked back to the very definition of these long-range interacting theories such as QED. The IR catastrophe and its resolution by cancelling the divergences will be discussed. The Bloch-Nordsieck model provides the deep IR description of QED, and in its framework all the radiative corrections to the electron propagator can be fully summed, giving the infra-particle interpretation to the electron. This solution can be obtained by using the functional technique based on the Schwinger-Dyson equation with the aid of the Ward-Takahashi identities. At finite temperatures we use the Keldysh description, that will give a matrix structure to the equation. Again an analytic solution can be obtained for the exact spectral function. The finite temperature solution provides a natural explanation of IR finiteness.